Monday, 23 April 2018

Otago Central Railway [61N]: Cromwell-Alexandra 11

I'm just opening this post with a note about the recently completed and published article series about the Otago Central Railway. There were some issues with editorial changes to the last part of the article. These are being addressed with a Research Note and a Letter to the Editor that will be published in a future edition of the NZ Railway Observer. Hopefully this will go some way to reconciling some of the issues related to the differences in use of imperial and metric distance measurements which have been highlighted in the above article series.

Work today is continuing to update the aerial footage from Cromwell to Clyde to the 0.4 metre higher resolution photography from Linz replacing the 0.75 metre photography that I originally used to do the aerial photos for the gorge. At the moment I am rendering the 0.75 metre intermediate tiles which consist of the lower resolution tiles with the higher resolution stuff overlaid on top of them. Once these tiles have been rendered they can be imported back into Gimp, after which the next stage is to copy the masks from the original plain tiles to the composites, and as they occupy exactly the same area this is a very straightforward 1:1 copy and paste of the masks.

Then once that has been completed then the final mosaic tiles, consisting of a mixture of 0.75 metre, 0.4 metre and historic aerial photography, are rendered to be loaded into Qgis as the the aerial photography backgrounds for this part of the maps. The overlaying of the 0.4 metre tiles over the 0.75 metre tiles went very smoothly and routinely. Once the first tile in the upper left corner (Cromwell township) was correctly positioned, all of the other tiles were anchored in position relative to this tile and all of the tiles were in perfect alignment at all times to the 0.75 metre tiles, as noted repeatedly when each new 0.4 metre tile was scaled down and positioned. The net result will be a higher quality background for the maps as the 0.4 metre coverage is much sharper and clearer as well as having more accurate colours (at least that's what I assume).

Because the intermediate rendering of the composite tiles prior to masking is as usual a slow process with Gimp, I am simultaneously using a second computer to update the maps in Alexandra in particular so that they are completed at the same time so hopefully I can start rendering out the maps in the next few days and sign off this section of the line as completed. As it can be noted that doing these historical aerial photos for the Cromwell Gorge section of the maps has been the most time consuming part of the whole project with a lot of different steps involved.

This image shows the Alexandra rail yard at Chicago St crossing using the 1965 aerial photo layer background. Qgis 2.18.18 is running in a virtual machine with the guest OS being Debian 9.4 running LXQT. Whilst I have lots of VMs running 2.18 this one happens to be the best functioning one right now.